29 May 2014

In a companion study to an SAE paper presented in April (earlier post), researchers at MIT have quantified the net economic and CO2 emissions benefit that could be obtained by utilizing 98 RON gasoline in light-duty vehicles, based on reasonable assumptions for possible refinery changes and the evolution of the LDV fleet. The paper, they note, is the first modern, peer-reviewed publication to address the costs and benefits of introducing higher octane gasoline.

According to the analysis, published in the ACS journal Environmental Science & Technology, greater use of 98 RON gasoline in appropriately tuned vehicles could further reduce annual gasoline consumption in the US by 3.0–4.4%. Even accounting for the increase in refinery emissions from production of additional high-RON gasoline, net CO2 emissions are reduced by 19–35 Mt/y in 2040 (2.5–4.7% of total direct LDV CO2 emissions). The MIT team estimated the annual direct economic benefit to be $0.4–6.4 billion in 2040, and the annual net societal benefit—including the social cost of carbon—to be $1.7–8.8 billion in 2040.

Reduction in CO2 emissions, direct monetary savings due to changes in fuel production and consumption, and net societal benefit (combining direct monetary savings with monetized value of CO2 emissions reduction based on the social cost of carbon) associated with switching to high usage of premium gasoline, and sensitivities of these benefits to changes in fuel prices, refinery configuration, and fuel specifications. Error bars indicate the estimated range of CR/ON. Credit: ACS, Speth et al. Click to enlarge.

In this paper, we examine the potential environmental and economic benefits of using higher-RON gasoline in new LDVs which use improved fuel characteristics to deliver improved fuel economy. The efficiency of spark ignition engines depends on the engine compression ratio and, for turbocharged engines, the boost level. The maximum allowable compression ratio and boost level are limited by the propensity of the fuel to autoignite before being consumed by the spark-initiated flame, a phenomenon referred to as knock. The antiknock characteristics of an engine can be expressed in terms of an octane requirement, where higher compression ratios and boost levels correspond to higher octane requirements, all other considerations being held equal. A fuel’s resistance to knock is characterized by its octane number, which is determined according to the standard research octane number (RON) and motor octane number (MON) test procedures. Thus, there is a direct relationship between the potential efficiency of a spark ignition engine and the octane number of the fuel.

… we assume a policy where the US adopts a RON standard for regular and premium gasoline, with minimum RON ratings that correspond approximately to those of existing grades, 92 RON for regular and 98 RON for premium. Then, vehicle manufacturers introduce new vehicles which require the 98 RON gasoline. Over time, these vehicles will make up an increasing fraction of the LDV fleet, and refineries will produce larger proportions of premium (high RON) gasoline to satisfy the fuel demand from these vehicles. By introducing vehicles that utilize higher-octane gasoline in this manner, a gradual increase in the average RON can be realized without requiring the introduction of a new fuel grade.

We note that the requirement of using only existing fuel grades places limits on possible efficiency improvements. Significant innovations in vehicle or fuel processing technology could warrant the introduction of additional automotive fuels in order to achieve system-wide economic and environmental benefits beyond those described in this study.

—Speth et al.

They used engine simulations (with Autonomie), a review of experimental data, and drive cycle simulations to estimate the reduction in fuel consumption associated with using higher-RON gasoline in individual vehicles. For the fleet model, they introduced four additional powertrains to represent the set of vehicles of each powertrain type designed to operate on high-RON gasoline. They assumed that higher-RON vehicles start entering the fleet in 2020, with 100% of vehicles sold for each powertrain type being the higher-RON type by 2030.

For the refinery side, they used an optimization-based modeling approach to estimate the changes in refinery operations as the ratio of premium to regular gasoline changes. They adjusted the model to represent a 100,000 barrel per day refinery with a product slate that approximates current US petroleum product consumption. In the baseline scenario, they assume that US regulators replace the current AKI standard for gasoline with a 92 RON minimum for regular gasoline and 98 RON minimum for premium gasoline. These values approximate the current RON values of regular and premium gasolines with AKI values of 87 and 93, respectively.

To estimate CO2 and economic impacts, they examined the changes in refinery operations required to shift from a low premium fraction (10% of gasoline production) to a high premium fraction (80% of gasoline production).

In the baseline scenario, total gasoline production decreases by 6.1%—partly due to the reduced yield associated with higher-severity reforming, and partly a result of diversion of streams from the gasoline pool to other products, in particular diesel and light naphtha.

Ethanol, they noted, has a significant impact on the ability of the refinery to produce large quantities of high-RON gasoline. They considered scenarios in which the amount of ethanol blended with gasoline was increased from 10% by volume to either 15% or 20% (E15 and E20). They found that minimal refinery adjustments are required to produce a high-premium product slate with E15 or E20—suggesting that higher ethanol blending would make increasing the RON of premium gasoline feasible.

Our analysis suggests that transitioning the US LDV fleet to higher-RON gasoline would result in significant economic and environmental benefits through reduced gasoline consumption.

Realizing these benefits would require coordination among auto manufacturers, refiners, and regulatory organizations. Auto manufacturers would need to modify engine designs to increase compression ratios and boost levels, and require that these vehicles be operated using higher-RON gasoline. Refiners would need to adjust the price differential between regular and premium gasoline to ensure that purchasing higher-efficiency vehicles requiring premium gasoline was an economical choice for consumers. Regulatory organizations would need to establish minimum RON requirements for premium gasoline. Our results indicate that the adoption of a RON standard for gasoline would be advantageous in conjunction with increased use of high-octane gasoline in reducing CO2 emissions and providing an overall societal economic benefit.

… Our results suggest that increased production of high-RON gasoline would make it possible to capture the octane number benefits of ethanol to reduce CO2 emissions while reducing fuel costs. Scenarios with larger levels of ethanol blending demonstrate the largest benefits in terms of both CO2 emissions reductions and economic savings. The projected reduction in total gasoline consumption means that the ethanol content of gasoline could be increased while maintaining the current level of total ethanol production, provided vehicles were designed to operate using such fuels. With E15, transitioning to premium gasoline would potentially reduce annual CO2 emissions by 31−49 Mt (worth $2.1−3.3 billion) depending on refinery configuration, and directly save $7.9−14.1 billion annually.